Transient electrical potentials measured during the unidirectional freezing of NaCl/H₂O solutions

An experimental study was carried out to investigate the electrical effects produced during the freezing of NaCl/H₂O solutions. For this purpose, a pair of platinum foil electrodes was inserted into the sample solution layer on the freezing stage of a cryomicroscope and connected to an electrometer. With this setup, the variation of freeze-induced potential with changes in initial solution concentration and ice-liquid interface propagation kinetics was investigated. Initial concentrations from 10^-6M to 0.154M NaCl were frozen using a variety of interface behaviors. The results reported by previous investigators were limited in range and not always consistent, but indicated that the potential produced resulted from a preferential incorporation of solute species, implying that the basic mechanisms were diffusion controlled. For a moving boundary problem with diffusion, the temperature and concentration profiles as well as the shape and position of the interface need to be fully specified. A careful control over all these factors was achieved by using a special freezing stage mounted on a cryomicroscope which permits simultaneous visual observation of the interface morphology and motion. It was generally observed that the potential exhibits a steady increase with time as the interface advances. The signal was found to be tied to the motion of the interface. Both the rate of increase of the potential and the magnitude of the maximum potential measured was observed to be related to the velocity of the interface. If the interface was stopped during a freezing protocol, the signal immediately began to decay. The voltage measured was found to be proportional to the calculated interface concentration. The highest voltages were measured at 10^-3M NaCl. Voltages at concentrations greater than 10^-3M were small in magnitude but still significant.